Magnetic recording media are widely used as sound-recording tapes, video tapes, computer tapes, disks, etc. In such magnetic recording media, recording densities are becoming higher and recording wavelengths are becoming shorter, and in addition to an analogue recording mode, a digital recording mode is being investigated. Although efforts are being made to attain higher output and lower noise in the trend toward higher densities and shorter recording wavelengths, these are mainly based on improvements in the magnetic property of ferromagnetic metal particles having excellent magnetic properties. Specifically, higher coercive forces are achieved by reducing the particle size of ferromagnetic metal particles or narrowing the particle size distribution, while higher saturated magnetic flux densities are achieved by adding a slight amount of a metallic element such as Co or Ni.
Reduction of the particle size in ferromagnetic metal particles and addition of a metallic element such as Co or Ni, enhance the oxidizability inherent in ferromagnetic particles, that is, they are disadvantageous in that the magnetic properties deteriorate over time.
Conventionally, the surfaces of metal particles have been gradually oxidized to form an oxide film in order to prevent oxidation over time. However, if the oxide film is made thicker by, for example, treatment with a peroxide, as described in JP-A-56-30707, so as to improve oxidative stability, this results in impaired magnetic properties, in particular, reduced saturated magnetic flux density, which detracts from the superiority of metal particles. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
Treating the surfaces of metal particles with a silane coupling agent, silicone oil, or the like, to make the surfaces hydrophobic is disclosed in, e.g., JP-A-52-155398, JP-A-53-5798, and JP-A-56-169304. However, this technique is disadvantageous in that the surfaces become so lipophilic that the particles have poor dispersibility.
Further, JP-A-61-223064, JP-A-61-223065, JP-A-61-2203066, JP-A-61-223067, JP-A-61-223068, JP-A-61-223069, and JP-A-61-223070 disclose surface treatment with a 2-thiouracil derivative, thiosalicylic acid, dithiouracil, 6-mercaptopurine, 2-mercaptobenzothiazole, and 6-amino-2-thiouracil, respectively. In addition, JP-A-4-6619 discloses an azimino compound (a compound having --NH--N.dbd.N-- in a cyclic structure), a quinone, naphthol, and an oxime, JP-A-4-6617 discloses a hydroxylated compound having a cyclohexane framework (skeleton), and JP-A-3-292617 discloses use of dihydroxynaphthalene or a derivative thereof; these compounds are regarded as the newest and most improved surface-treating agents for eliminating the problems described above.
In magnetic recording media employing ferromagnetic metal particles the magnetic layer may have insufficient head-touching properties and, hence, when the magnetic layer is run on a magnetic head at a low speed, the surface of the magnetic head may be fouled during running, resulting in decreased output. This magnetic-head fouling is sometimes called seizing because it is a phenomenon similar to metal seizing. This problem has been severe, especially under low-humidity conditions. The compounds disclosed in the prior art references cited above are mainly intended to improve dispersibility of magnetic particles. Often, adhesive debris from the magnetic layer deposit on the magnetic head to cause head fouling or fill-up gaps in the magnetic head to cause head clogging and thereby impair electromagnetic characteristics. However, the prior art contains no descriptions therein concerning the effect of preventing these problems.
The incorporation of an abrasive material into a magnetic layer for the purpose of diminishing clogging and head fouling may cause excessive head abrasion which considerably decreases output to a degree which may be unacceptable in practical use.
A technique for adhering a metal, such as Cr, Mn, or Zn, or a metal oxide thereof, to ferromagnetic metal particles so as to improve oxidative stability is disclosed in, e.g., JP-A-50-41097, JP-A-51-112465, JP-A-49-41899 and JP-B-44-27942. (The term "JP-B" as used herein means an "examined Japanese patent publication.") However, this technique is disadvantageous in that surface properties are changed and this may result in poor dispersibility and impaired magnetic properties.
In order to eliminate problems concerning oxidative stability and dispersibility, and to impart abrading properties to a magnetic layer, Al or a compound thereof has been adhered to ferromagnetic metal particles, as disclosed in JP-B-63-22366, JP-A-63-103423 (combination with a polar-group binder), and JP-A-63-103424 (combination with a specific abrasive material). However, while increasing the amount of Al present on the ferromagnetic metal particles enhances the abrading properties of the tape, which eliminates the problem of head fouling, it may cause increased head abrasion.
To sum up, the prior art techniques described above have failed to satisfy the contradictory requirements for reducing head abrasion and preventing head fouling.
Consequently, there is a desire for a magnetic recording medium in which surface-treated ferromagnetic metal particles having increased oxidative stability are excellently dispersed in a binder resin, the particles having reduced abrasion with respect to magnetic heads and no magnetic-head fouling even under low-humidity conditions.